This invention relates to pipeline pigs.
Devices know as pigs are commonly passed through pipelines in order to service or inspect the pipeline. A pig may contain the necessary servicing or inspection equipment or may draw other devices behind it. In pipelines for conveying fluids, the pig is commonly driven along the pipeline by the flow of the pipeline fluid. To achieve this the pig may be fitted with one or more flexible disks or cups attached to the body of the pig and extending outwardly to the interior pipe wall so as to form a sufficient seal in the pipeline. Such arrangements have been known and used for many years. If the seal is substantially complete, the pig will travel at the speed of the pipeline fluid.
In gas pipelines in particular, it can be advantageous for the pig to travel more slowly than the speed of the gas. This can arise when the pipeline operator wishes to maintain a high value gas flow in the pipeline in order to supply customers and maintain income while running pigs that may operate most effectively at lower speeds. Inspection pigs generally have a limit on their maximum speed above which the inspection performance is degraded or impaired. Similar considerations can also apply to cleaning pigs. Unnecessarily slow pig speeds are also desirably avoided. Accordingly, a pipeline pig is likely to have an optimum operating speed window.
In order to be able to reduce the speed of the pig, it has been proposed to provide a pig with a gas bypass. British Patent No. 2097537 discloses an example in which gas flow through an annular array of ducts in the body of the pig is regulated by controlling the number of ducts that are open at any time, by opening or closing each duct by a butterfly valve operated by a corresponding actuator, in response to an error signal derived from a comparison of the sampled speed of the pig with stored upper and lower speed threshold values.
U.S. Pat. No. 5,208,936 discloses a similar annular arrangement of ducts that are simultaneously adjusted for effective flow area by a rotatable orifice plate at one end. Spaced apart orifices in the plate are more or less aligned with the ducts by rotation of the plate by a stepper motor in response to the speed of the pig as measured by a pickup wheel.